It is often desirable that electronic circuits be adjusted or re-configured after manufacture, packaging, or assembly. One technique often used for adjustment or reconfiguration of an electronic circuit is the use of fuses, which are electrical connections which may be disconnected by “blowing” a fuse. Blowing the fuse turns a low-resistance (conductive) electrical connection into an open-circuit.
A common application of fuses is in the adjustment of resistance within an analog circuit. Fuse-adjustable resistances may be implemented by an array of elemental resistors, where a subset of the array remains connected (in series or parallel) to form the resulting resistance after blowing a number of fuses. The particular fuses blown, and arrangement of remaining resistor elements, determine the final (adjusted) resistance.
FIG. 1a shows a common arrangement implementing a fuse-adjustable resistance, having 6 bits of precision. The total resistance in this example is 64 R, while the smallest unit of resistance change is R, which is 26 times smaller than the maximum resistance. FIG. 1b shows a general example of a series-connected fuse-adjustable resistance, having 6 fuses in order to independently (selectively, in a random-access manner) include 6 different resistances R1-R6. FIG. 1c shows a general example of a parallel-connected fuse-adjustable resistance, having 3 fuses to independently (selectively, in a random-access manner) disconnect 3 different resistances R1-R3. Since in each of these circuits the fuse-adjustable resistance is a two-terminal circuit element, a “terminal 1” and a “terminal 2” are shown for each circuit. Note that addressing such fuses typically requires direct access to the fuses themselves (i.e. both ends of each fuse, as illustrated in FIGS. 1a, 1b, 1c).
Since resistances (and also capacitors, inductors) are in principle passive devices, it is desirable to have a fuse-adjustable resistor (or capacitor/inductor) whose fuse-blowing circuitry is passive, not requiring active circuitry, in order to avoid additional process requirements which would be required for the creation of active circuitry.